The instant invention relates to fiber lasers, and more particularly to a high-power seed fiber laser oscillator having a narrowed spectral linewidth.
A number of laser applications require lasers with a narrow spectral linewidth. Further, a number of the narrow linewidth applications, such as pumping of gain media with a narrow spectral absorption line, also require high-power spectral density coupled into the media. A narrow spectral linewidth is important for example, to single frequency laser systems. The narrower the spectral linewidth, the higher the spectral density.
The term “linewidth” is applied here to describe an envelope of spectral emissions which is defined by a distribution of longitudinal modes (frequencies). Fundamentally, a laser emits a plurality of closely spaced longitudinal modes, which are centered about a particular frequency. However, at a discrete moment in time, only one of those discrete modes exists. When viewed in time, the longitudinal mode of the laser jumps from one mode to another defining an envelope of emissions. The amount of time the laser operates in one of these modes, along with a measure of intensity during that time, defines a power of the laser (power=energy/time).
One type of narrow linewidth laser is a fiber laser. A fiber laser is defined as a laser with an optical fiber as the gain media. In most cases, the gain medium is a fiber doped with rare-earth ions such as erbium, neodymium, ytterbium, thulium or praseodymium. Each of these rare-earth ions absorbs light at one wavelength and emits light at another (usually longer) wavelength. For example, erbium is usually pumped at 980 nm and emits light at 1550 nm. One or more multi-mode laser diodes are used for pumping of the doped fiber and the resonant cavity is formed by Bragg gratings written directly into the fiber of the system or by fiber loop mirrors. The result is a narrow linewidth single longitudinal mode optical signal. Simple fiber laser systems provide a very stable, narrow linewidth optical signal. However, they are somewhat limited in power.
FIG. 1A illustrates a rather typical configuration of a single-frequency fiber laser system 10 including a master oscillator or seed laser 12 and one, or more, amplifier stages (amplifier cascades) 14. This configuration is also referred to as a Master Oscillator Fiber Amplifier or MOFA configuration. In order to increase the threshold of non-linear effects downstream in the system, the seed laser 12 is configured to radiate at a relatively low-power output (only up to a few hundred milliwatts). Generally, the seed laser 12 is configured with an active gain fiber 16, a pump source 18, and a pair of reflectors 20, 22 defining an optical cavity that includes the active gain fiber 16. The active fiber is doped with erbium in the illustrated configuration and is pumped with multi-mode laser diode arrays. As noted above, the active dopant may alternatively comprise any of the known rare-earth ions. The reflector 22 is illustrated as a tunable fiber grating, but could also be configured with a regular Bragg fiber grating for a fixed wavelength. An optical isolator 24 is located on the output end of the seed laser to prevent feedback.
To increase the output of the system, the single frequency system typically includes a few amplifying stages (cascading amplifiers) 14. However, the overall increased length of the single frequency system due to the amplifying stages, is associated with the occurrence of undesirable non-linear effects, such as Brillouin scattering. Another cause for the occurrence of Brillouin scattering is an overly narrow spectral linewidth, which is also associated with the configuration as shown.
FIGS. 1B, 1C and 1D illustrate operation of the system 10 of FIG. 1A. In the illustrated example, the erbium fiber 16 is pumped by multi-mode pump diodes 18 at a pump wavelength of 975 nm. The pump light stimulates an optical emission from the erbium fiber at a wavelength of about 1548 nm.
FIG. 1B is a screen shot of the output of the system on an optical spectral analyzer (OSA) showing the dependence of the power density (power/area) (spectral emission envelope 28) from an optical frequency or wavelength. The envelope shows a fairly narrow spectral linewidth 27 centered about 154 μm. FIG. 1C is a graphical illustration of the envelope 28 of FIG. 1B, showing the discretely pulsed modes 26 that define the envelope 28. Referring to FIG. 1C, at any given moment of time, seed oscillator 12 generates a single frequency (mode) 26 oscillating between the reflectors 20, 22. As described above, the laser actually emits a plurality of closely spaced longitudinal modes 26 (only few are shown) which are centered about a particular frequency (in this case 1548 nm). When viewed in time, the emission of the laser 10 jumps from one mode 26 to another 26 defining the envelope 28, i.e. the mode 26 exists in only one frequency for only a discrete period of time, and then jumps to another mode. The OSA integrates the fluctuation of this single spectral line or mode 26 within envelope 28 and shows it graphically as in FIG. 2B.
Under certain conditions of the resonant cavity, spectral lines (modes) 26, which are close to one another in time, are “synchronized” within the cavity and produce peaks 30 of power at discrete moments in time. Looking back at the formula power=energy/time, there is more energy measured in the same period of time because of the synchronization. These peaks 30, corresponding to the respective pulses or synchronization of the modes, can be seen in the screen shot of FIG. 1D and are generally known as spectral beating. Thus, although the seed oscillator 12 has a continuous wave configuration, it effectively operates in a quasi-pulsed regime. These peaks 30 are highly undesirable in laser systems. Each peak 30 is characterized by a substantial instantaneous power surge capable of damaging fiber and fiber components and of causing the appearance of non-linear effects in amplifiers 14 at undesirably low thresholds. Accordingly, the seed laser 12 can only be operated at a power such that the periodic power surges 30 do not cross the damage threshold of the fiber and components. However, this now leads to another issue in the design of the system. The seed signal may not be strong enough that the amplifier 14 can adequately amplify the narrow spectral line 26.
Turning to FIG. 2A, it is generally known that a decrease of the amplitude of peaks 30 can be achieved by broadening of the linewidth 27 of the laser (widening of the envelope). The broader the linewidth, the smaller the power density about the desired frequency, and the lower the amplitude of peaks 30. FIG. 2A illustrates a MOFA 10 having substantially the same configuration as in FIG. 1A, but which realizes a broadening of the linewidth by employing an additional length of fiber 32 (extension loop). FIG. 2B shows a broadening of the linewidth with the same signal amplitude. FIG. 2C graphically shows that the extension loop 32 is operative for creating additional modes 26, as compared to FIG. 1C, across the envelope and generally results in fewer occurrences of “synchronization” of the modes 26. FIG. 2D shows that the system of FIG. 2A exhibits a marked reduction in both the number of peaks 30 and the amplitude of those peaks 30. However, the undesirable peaks 30 still lower the threshold for the occurrence of non-linearities in the downstream amplifiers. Further broadening of the linewidth would be possible by adding further extension loops, physically lengthening the system. However, further broadening of the linewidth reduces the spectral density at the desired frequency and reduces the effectiveness of a system that requires high spectral density at a narrow spectral absorption line.
Accordingly, there is a need for an improved fiber laser system which operates to reduce the intensity of modal beating (power peaks) without limitlessly curtailing the desired output power of the seed laser.
The instant invention provides a high-power seed fiber laser oscillator which reduces the intensity of modal beating without further broadening of the spectral line and thus permits an increase in the power of the seed laser without introducing non-linearities in the downstream amplifiers.
The instant invention accomplishes this goal by introducing at least one additional resonator cavity into the system. It is believed that the addition of a second resonator cavity (or perhaps multiple resonator cavities) increases the number of discrete longitudinal modes (frequencies), (although still centered about a central frequency) thereby decreasing the time that any one mode can exist and decreasing the chances of synchronizing any of the modes at any time within the system. The result is a reduction in the number of modal peaks and a reduction in the amplitude of the peaks that are induced.
More specifically, the high-power narrowed-linewidth fiber laser system of the present invention includes a seed oscillator with multiple resonant cavities and at least one amplifier stage. Because of the reduction in the number and amplitude of the peaks, the seed laser can be driven at a higher power, reducing the need for downstream amplifiers.
The seed oscillator includes a single mode, rare-earth doped, active gain fiber, a bi-directional pump source to introduce pump light into the active gain fiber, and a single-mode output fiber arranged at one end of the active gain fiber. A tunable fiber Bragg grating is preferably provided within the single mode output fiber followed by an optical isolator to prevent feedback from downstream amplifier stages. A 50/50 coupler is provided at the other end of the active fiber to split light from the active gain fiber into first and second branches, which form the two separate resonant cavities. Each of the branches from the coupler is terminated with a fiber loop mirror. Accordingly, a first resonant cavity including the active gain fiber is formed between the output Bragg grating and the fiber loop mirror on the first branch, and a second resonant cavity including the active gain fiber is formed between the output Bragg grating and the fiber loop mirror on the second branch. The first resonator cavity also includes an extension loop so that the resonant cavities have different optical lengths. The second resonant cavity creates a different set of longitudinal modes, although still centered about the same frequency, and thus minimizes the synchronization of all modes within the system to thereby reduce non-linear effects prior to amplification.
The amplifier stage(s) can comprise any known amplification system, but preferably within the context of the invention, the amplifier stage includes an active multimode gain fiber capable of supporting a substantially single fundamental mode at the signal wavelength, wherein the single mode output fiber of the seed oscillator and the multimode gain fiber of the amplifier are mode-matched and coupled without a mode converter.
Accordingly, among the objects of the instant invention are:
the provision of an improved high-power seed fiber laser oscillator which reduces the intensity of modal beating without further broadening of the spectral linewidth;
the provision of an improved seed fiber laser which permits an increase in the power of the seed laser without introducing non-linearities in the downstream amplifiers;
the provision of an improved seed fiber laser which introduces at least one additional resonator cavity into the system to increase the number of longitudinal modes within the spectral distribution envelope; and
the provision of such an improved seed fiber laser which minimizes the occurrence of synchronization of the longitudinal modes, resulting in a reduction in the number of modal peaks and a reduction in the amplitude of the peaks that are induced.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.